"I want to inform people that in January I will not continue," Martinez told TyC Sports. "It's been a very difficult decision for me, but my journey continues. I think I've given people what they deserve, and that is everything for those who supported me."

Atlanta have reportedly agreed to pay the Argentina international's $17 million release clause.

Asked why he had chosen Atlanta, Martinez said: "It's the club that has put its trust in me. European clubs did not put their trust in me at the time, and I thought long and hard about it with my family and my agent.

"The MLS is a league that has grown a lot, and I hope I can keep on making history at that club."

River's win sees them qualify for the Club World Cup, which will kick off in the United Arab Emirates later this week.

"I will play at the Club World Cup," Martinez said. "To experience these moments with this club is the most beautiful thing in the world. There are many things I achieved with this club, and I'm going to miss it a lot."

River captain Leo Ponzio, meanwhile, said he hoped to come up against defending champions Real Madrid in the final.

"I wish that the Club World Cup final will be River vs. Real Madrid," he told Cadena Ser radio. "We know who they are, how they play and it would be something to enjoy."

Atlanta United won the title in only their second season on Saturday when they beat Portland Timbers 2-0 in the MLS Cup playoff final.